USA — Flooding that followed last summer’s Schultz Fire on the flanks of the San Francisco Peaks near Flagstaff should be a warning to residents of all areas downstream from Arizona’s forested mountains, geologists say.
“After a fire, the hydrology on the watershed changes significantly, and it can take just a normal garden-variety or wimpy little storm to produce big floods and debris flows,” said research geologist Ann Youberg of the Arizona Geological Survey.
Youberg worked with scientists from the U.S. Forest Service to document the causes and effects of floods that floated boulders down stream channels, carved gullies and flooded homes in rural subdivisions northeast of Flagstaff in July and August last year.
She said rainstorms of a severity that could be expected every 25 years sent water into homes outside of mapped flood plains after fire denuded 15,075 acres of forested watershed in late June.
The flooding affected 120 homes, said Coconino County Emergency manager Sherrie Collins, and four were significantly damaged.
On July 20, flooding also caused the death of a 12-year-old girl who was washed away while watching the floods with her younger sister.
In Arizona, where the rainy season quickly follows the hottest and driest months, there is little or no time to prepare for post-fire flooding, Youberg said.
The article, authored by Youberg along with Karen Koestner and Dan Neary of the Forest Service’s Rocky Mountain Research Station in Flagstaff, is published in the winter edition of Arizona Geology, the online newsletter of the Arizona Geological Survey.
This is not the first example of post-fire catastrophe in Arizona, the report says, and it should “remind us once again of the need to consider potential post-fire impacts to existing communities and future developments now, while there is time to plan and implement mitigation measures.”
“Arizona communities in the urban-wildland interface urgently need building and flood ordinances that anticipate wildfires and potential post-fire events,” the report concludes.
Pima County has recently seen evidence of fire’s ability to alter watersheds, Youberg said.
Flooding along the Cañada del Oro after the 2003 Aspen Fire in the Santa Catalina Mountains led the county to buy out 63 homeowners at a cost of $10.2 million, removing those homes from the path of future floods.
In Coconino County, “flooding was surprisingly widespread, extending into the community of Timberline west of U.S. 89 . . . and ultimately inundating low-lying areas in the Doney Park development four miles from the burn,” the report says.
Collins said the area is protected by “32,000 linear feet of concrete highway barriers and 300,000 sandbags” as the county gears up for “the possibility of warm rain on snow” at winter’s end.
The Arizona Geology report says the area will remain at risk until ground cover re-establishes on the burned slopes, something that can take three to five years.
Collins said an engineering study the county commissioned predicts even more years of jeopardy.
“I think this is a great wake-up call for any community,” Collins said.
“The risk is definitely there for our urban-wildland interface community,” she said. If this fire had burned to the south and southwest, it would have unleashed more damaging floods on the more populated areas of Flagstaff, Collins said.
She said the residents of these rural subdivisions did a good job of creating defensible space, and firefighters were able to protect them from the fire. None of them, however, had flood insurance – a precaution she recommends for all homeowners.
“Even though you have your home protected and it’s safe from the fire doesn’t mean it’s not at risk,” she said.